Industrial hydrogenation reactions are often performed by using finely divided powdered slurry catalysts in stirred-tank and reactors. These slurry phase reaction systems are inherently problematic in chemical process safety, operability and productivity. The finely divided, powdered catalysts are often pyrophoric and require extensive operator handling during reactor charging and filtration. By the nature of their heat cycles for start-up and shut-down, slurry systems promote co-product formation which can shorten catalyst life and lower yield to the desired product.
An option to the use of finely divided powder catalysts in stirred reactors has been the use of pelleted catalysts in fixed bed reactors. While this reactor technology does eliminate much of the handling and waste problems, a number of engineering challenges have not permitted the application of fixed bed reactor technology to the hydrogenation of many organic compounds. Controlling the overall temperature rise and temperature gradients in the reaction process has been one problem. A second problem is that in fixed bed packed reactors there is a significant pressure drop due to the high flow rates required for hydrogenation. A third problem is that liquid-gas distribution is problematic thus often leading to poor conversion and localized concentration gradients. A fourth problem is that the product water phase in a two liquid phase system tends to block access of the reactant to the active catalyst sites and thereby decrease the reaction rate or, in the alternative, result in inconsistent reaction rates.
Monolith catalytic reactors are an alternative to fixed bed reactors and have a number of advantages over conventional fixed bed reactors. These reactors have low pressure drop which allow them to be operated at higher gas and liquid velocities. These higher velocities of gas and liquids promote high mass transfer and mixing and the parallel channel design of a monolith inhibits the coalescence of the gas in the liquid phase.
Monolith catalytic reactor development has been an ongoing process in an effort to enhance catalytic activity and catalyst life. Exposure of the catalytic metal in the monolith catalytic reactor to the reactants is necessary to effect good reaction rates. However, efforts to enhance exposure of the catalytic metal often are at odds with enhancing adhesion of the metal to the monolith substrate. Embedding the catalytic metal in a coating applied to the surface of the monolith may result in greater adhesion of the catalytic metal but also reduces catalytic activity.
The following articles and patents are representative of catalytic processes employing monolith catalysts and processes in chemical reactions including the hydrogenation of nitroaromatics and other organic compounds.
Hatziantoniou, et al. in xe2x80x9cThe Segmented Two-Phase Flow Monolithic Catalyst Reactor. An Alternative for Liquid-Phase Hydrogenationsxe2x80x9d, Ind. Eng. Chem. Fundam., Vol. 23, No.1, 82-88 (1984) discloses the liquid phase hydrogenation of nitrobenzoic acid (NBA) to aminobenzoic acid (ABA) in the presence of a solid palladium monolithic catalyst. The monolithic catalyst consisted of a number of parallel plates separated from each other by corrugated planes forming a system of parallel channels having a cross sectional area of 1 mm2 per channel. The composition of the monolith comprised a mixture of glass, silica, alumina, and minor amounts of other oxides reinforced by asbestos fibers with palladium metal incorporated into the monolith in an amount of 2.5% palladium by weight. The reactor system was operated as a simulated, isothermal batch process. Feed concentrations between 50 and 100 moles/m3 were cycled through the reactor with less than 10% conversion per pass until the final conversion was between 50% and 98%.
Hatziantoniou, et al. in xe2x80x9cMass Transfer and Selectivity in Liquid-Phase Hydrogenation of Nitro Compounds in a Monolithic Catalyst Reactor with Segmented Gas-Liquid Flowxe2x80x9d, Ind. Eng. Chem. Process Des. Dev., Vol. 25, No.4, 964-970 (1986) discloses the isothermal hydrogenation of nitrobenzene and m-nitrotoluene dissolved in ethanol using a monolithic support impregnated with palladium. The authors report that the activity of the catalyst is high and therefore mass-transfer is rate determining. Hydrogenation was carried out at 590 and 980 kPa at temperatures of 73 and 103xc2x0 C. Again, less than 10% conversion per pass was achieved. Ethanol was used as a cosolvent to maintain one homogeneous phase.
U.S. Pat. No. 6,005,143 discloses a process for the adiabatic hydrogenation of dinitrotoluene in a monolith catalyst employing nickel and palladium as the catalytic metals. A single phase dinitrotoluene/water mixture in the absence of solvent is cycled through the monolith catalyst under plug flow conditions for producing toluenediamine.
U.S. Pat. No. 4,743,577 discloses metallic catalysts which are extended as thin surface layers upon a porous, sintered metal substrate for use in hydrogenation and decarbonylation reactions. In forming a monolith, a first active catalytic material, such as palladium, is extended as a thin metallic layer upon a surface of a second metal present in the form of porous, sintered substrate. The resulting catalyst is used for hydrogenation, deoxygenation and other chemical reactions. The monolithic metal catalyst incorporates catalytic materials, such as, palladium, nickel and rhodium, as well as platinum, copper, ruthenium, cobalt and mixtures. Support metals include titanium, zirconium, tungsten, chromium, nickel and alloys.
U.S. Pat. No. 5,250,490 discloses a catalyst made by an electrolysis process for use in a variety of chemical reactions such as hydrogenation, deamination, amination and so forth. The catalyst is comprised of a noble metal deposited, or fixed in place, on a base metal, the base metal being in form of sheets, wire gauze, spiral windings and so forth. The preferred base metal is steel which has a low surface area, e.g., less than 1 square meter per gram of material. Catalytic metals which can be used to form the catalysts include platinum, rhodium, ruthenium, palladium, iridium and the like.
EPO 0 233 642 discloses a process for the hydrogenation of organic compounds in the presence of a monolith-supported hydrogenation catalyst. A catalytic metal, e.g., Pd, Pt, Ni, or Cu is deposited or impregnated on or in the monolith support. A variety of organic compounds are suggested as being suited for use and these include olefins, nitroaromatics and fatty oils.
There is a report by Delft University, in Elsevier Science B.V., Preparation of Catalysts VII, p. 175-183 (1998) that discloses a carbon coated ceramic monolith where the carbon serves as a support for catalytic metals. Ceramic monolith substrates were dipped in furfuryl alcohol based polymer forming solutions and allowed to polymerize. After solidification the polymers were carbonized in flowing argon to temperatures of 550xc2x0 C. followed by partial oxidation in 10% O2 in argon at 350xc2x0 C. The carbon coated monolith substrate typically had a surface area of 40-70 m2/gram.
The present invention relates to an improved process for the hydrogenation of an immiscible mixture of an organic reactant in water. The two phase immiscible mixture can result from the generation of water during the hydrogenation reaction itself or, by the addition of water to the reactant prior to contact with the catalyst or to the reactor. The improvement resides in effecting the hydrogenation of a two phase immiscible mixture of organic reactant in water in a monolith catalytic reactor comprised of a monolith support and a catalytic metal and having from 100 to 800 cells per square inch (cpi). This is accomplished by passing a two phase immiscible mixture of organic reactant in water through the reactor at a superficial velocity of from 0.1 to 2 m/second in the absence of a cosolvent for the two phase immiscible mixture.
The invention also relates to an improved monolith support comprised of a substrate having a polymer network/carbon coating applied to its surface, and, also, to an improved monolith catalytic reactor comprised of the monolith support and a catalytic metal, preferably a transition metal catalyst.
Several advantages are achievable in the process through the use of a monolith catalytic reactor and these include:
an ability to effect liquid phase hydrogenation of organic compounds as an immiscible phase in water and in the absence of a cosolvent;
an ability to obtain high throughput of product through the catalytic unit even though the reaction rate may be less than that using a cosolvent;
an ability to generate a monolith support suited for impregnation with a variety of catalytic metals and thereby forming a monolith catalytic reactor having excellent activity;
an ability to effect hydrogenation reactions at a consistent reaction rate; and, an ability to hydrogenate organic reactants under liquid phase conditions that permit ease of separation of reactants and byproduct.
The present invention relates to an improved process for the hydrogenation of an immiscible mixture (two phases) of an organic reactant in water. The immiscible mixture can result from the generation of water during the hydrogenation reaction or, if desired, by the addition of water to the reactant prior to or during the hydrogenation reaction.
There are numerous categories of organic compounds having functional groups that may be hydrogenated as a two phase mixture. The functional groups include nitro, anhydride, and the reaction product of a ketone or aldehyde and ammonia, aromatic amine, primary or secondary amine. The following are hydrogenation reactions involving these functional groups that co-produce water and can be hydrogenated in a monolith catalytic reactor.
Nitro Group Reduction
RNO2+3H2xe2x86x92RNH2+2H2O
where R is aromatic. Many nitro aromatics are capable of undergoing the hydrogenation reaction described by the process of this invention. Typical nitroaromatics are nitrobenzene, nitrotoluenes, nitroxylenes, nitroanisoles and halogenated nitroaromatics where the halogen is Cl, Br, I, or F.
Anhydride Reduction to Lactone or Ether 
Anhydrides such as maleic anhydride and phthalic anhydride are first hydrogenated to xcex3-butyrolactone and phthalide respectively. The xcex3-butyrolactone can be further reduced to tetrahydrofuran.
Reductive Alkylation or Reductive Amination 
When an aldehyde or a ketone is treated with ammonia or a primary or secondary amine in the presence of hydrogen and a hydrogenation catalyst, reductive alkylation of ammonia or the amine or reductive amination of the carbonyl compound takes place. R and Rxe2x80x2 can be aromatic or aliphatic. Examples of aldehydes and ketones useful in the hydrogenation reactions include formaldehyde, cyclohexanone and methyl isopropyl ketone. Reaction products resulting from the reaction of these aldehydes and ketones with primary and secondary amines include N-methylcyclohexylamine, N-methyldicyclohexylamine, N,N-dimethylcyclohexylamine, N-ethylcyclohexylamine, dicyclohexylamine, N,N-diethylcyclohexylamine, N,N,Nxe2x80x2-trimethylaminoethylethanolamine, N-ethyl-1,2-dimethylpropylamine and N,N,Nxe2x80x2,Nxe2x80x2-tetramethylpropanediamine.
By immiscibility of the reaction system leading to the presence of two phases, it is meant that two liquid phases are present at the operating temperature. The solubility of the organic reactant in water is not only a function of temperature but also a function of the solubility of the reaction product(s) with the organic reactant and with water. In some hydrogenation reaction systems, e.g., the hydrogenation of dinitrotoluene, the dinitrotoluene reactant, the toluenediamine reaction product and water produce essentially one liquid phase at stoichiometric reaction conditions of 60% toluenediamine, 39% water and 1% dinitrotoluene. In the hydrogenation of nitrobenzene, however, the reaction products of nitrobenzene, aniline and water, on the other hand, remain as a two phase system throughout the hydrogenation process. The following solubility data is for aniline in water and nitrobenzene in water at different temperatures.
Monolith catalysts, or sometimes referred herein as monolith catalytic reactors, employed herein consist of a monolith support which is based upon an inorganic porous substrate, a metallic substrate or a carbon based substrate. Sometimes the surface of the monolith substrate may be modified, as for example, with a coating derived from a carbon or a heat treated network polymer and thereby form a monolith support having a modified substrate. Often the monolith catalytic reactors are based upon a honeycomb of long narrow capillary channels, circular, square or rectangular, whereby gas and liquid are co-currently passed through the channels under a laminar flow regime. The flow of gas and liquid in these confined channels and under these conditions promotes xe2x80x9cTaylorxe2x80x9d flow with bubbles of gas, typically H2, squeezing past the liquid. This capillary action promotes very high initial gas-liquid and liquid-solid mass transfer.
The pressure drop within an effective monolith catalytic reactor can range from 2 kPa/m to 200 kPa/m for combined gas/liquid superficial velocities between 0.1 to 2 meters/second for 50% gas holdup in a monolith catalytic reactor having 400 cpi (cells per square inch). Typical dimensions for a honeycomb monolith catalytic reactor cell wall spacing range from 1 to 10 mm between the plates. Alternatively, the monolith catalytic reactor may have from 100 to 800, preferably 200 to 600 cpi. Channels or cells may be square, hexagonal, circular, elliptical, etc. in shape. (For purposes of convenience, it is assumed a monolith catalytic reactor comprised of the monolith support, whether a substrate or a network polymer containing including the catalytic metal, has the same cpi as the monolith substrate itself.)
Catalytic metals suited for the hydrogenation of water immiscible organics are impregnated or directly coated onto the monolithic substrate, a modified substrate or a washcoat which has been deposited onto the monolith. The catalytic metals include those Group VIb, Group VIIb, Group VIII, and Group Ib metals of the periodic table and conventionally used in hydrogenation reactions. Examples of catalytic metal components include rhodium, cobalt, Raney or sponge nickel, palladium, platinum, copper, ruthenium, rhenium and so forth. Often a mixture of metals are employed, one example being a mixture of palladium and nickel. For a monolith catalytic reactor where the monolith support is impregnated with a washcoat, the composition of catalytic metals is typically identified as a weight percent within the washcoat itself. The washcoat may be applied in an amount of from 1 to 50% of the monolith total weight. Typical catalyst metal loadings, then, range from 0.1 to 25% by weight and preferably from 1 to 10% by weight of the washcoat. The catalytic metals may be incorporated into or onto the surface of the monolith support including a coated or modified substrate in a manner generally recognized by the art. Incipient wetness from a salt solution of the catalytic metal is one example of a method for incorporating a metal catalytic component on the monolith support or modified (coated) monolith support.
The superficial liquid and gas velocities in the monolith channels are maintained to effect a desired conversion, e.g., 1% to 99% per pass. Typically, the superficial velocity through the monolith ranges between 0.1 to 2 meters per second with residence times of from 0.5 to 120 seconds.
Although not intending to be bound by theory, when a monolith support is used as a catalyst support, the morphology of the surface of the monolith support is important in order to (a) attach the active metal for hydrogenation for enhanced adhesion and (b) in the case of two immiscible liquid phases to permit selective adsorption of the reactant over the other immiscible phase, water, and the product for enhanced reaction rate.
In terms of a support for the catalytic metal, particularly a polymer network/carbon coating or carbon film carried on a substrate and thereby acting as a monolith support for the catalytic metal, eliminating micro porosity of the surface of the carbon coating or carbon film is advantageous for producing a monolith catalytic reactor having excellent activity and catalyst life. Small and medium size pores in the surface of the coating tend to lead to catalyst deactivation through pore plugging with high molecular weight co-products. Therefore, the carbon monolith support, a carbon coated substrate forming the monolith support or a polymer network/carbon coated substrate resulting in a monolith support should have a very low surface area for optimum activity, i.e., measured by adsorption of N2 or Kr using the BET method of from approximately 0.1 to 15 m2/gram of surface area.
To achieve the preferred polymer network/carbon coated monolith support having low surface area for use in forming the monolith catalytic reactor, polymer coating solutions are applied to the wall surface of the substrate and heated to a temperature below traditional carbonization temperatures. Examples of polymer forming solutions suited for producing polymer network/carbon coating include furfuryl alcohol solutions and solutions of furfuryl alcohol with other additives such as pyrrole and polyethylene glycol methyl ether. The furfuryl alcohol solutions may also be based upon prepolymers containing polymerized units of furfuryl alcohol. A preferred example is a furfuryl alcohol polymer solution derived from a furfuryl alcohol/pyrrole/polyethylene glycol methyl ether solution. An example of a copolymer is one based upon furfuryl alcohol and formaldehyde. Other examples include epoxy resins with amines; epoxy resins with anhydrides; saturated polyester with glycerol or other multifunctional alcohols; oil-modified alkyd saturated polyesters, unsaturated polyesters; polyamides; polyimides; phenol/formaldehyde; urea/formaldehyde; melamine/formaldehyde and others. Preferred polymer network/carbon coatings are based upon commercially available oligomers and copolymers of furfuryl alcohol as the coating solution.
The polymer coating solution is applied to the monolith substrate as a thin film such that the interior dimensions of the cells in the monolith support are not altered significantly. It remains desired to have cell dimensions of the monolith support and thereby the monolith catalytic reactor within the 100 to 800 cpi range.
Carbonization of the polymer coating is effected at relatively low temperature in an effort to effect adhesion of the polymer network/carbon coating. Temperatures for carbonization in producing the unique polymer network/carbon coatings range from 250 to 350xc2x0 C. vs. 550-900xc2x0 C. commonly used for these polymer solutions in the prior art. Because of the lower carbonization temperatures used herein, network polymers having polar groups, particularly those based upon furfuryl alcohol, will retain some of their functionality and are more like the polymer than carbon. These functional groups also can be coupled through reaction chemistry to anchor homogeneous catalysts, homogeneous chiral catalysts or ligands to the polymeric surface.
Hydrogenation of organic compounds is effected at temperatures of 60-180xc2x0 C. The hydrogenation pressure can be up to 1600 psig.